There are a number of different applications for sensing devices capable of sensing specified conditions in fluid mediums which indicate a situation to be detected. One such device is a water safety warning apparatus examples of which are described in U.S. Pat. No. 3,636,544, issued Jan. 18, 1972 to George Jorge Codina, and U.S. Pat. No. 3,953,843, issued Apr. 27, 1976 also to George Codina. Another such device is the standard smoke detector.
As a practical matter, these sensing devices should be simple and efficient in operation. In addition, as is particularly true with the swimming pool alert, they may be subject to substantial abuse so that they should also be extremely durable. Furthermore it is important that these devices be inexpensive to operate and relatively maintenance free.
From a mechanical standpoint both of the Codina structures mentioned above have several drawbacks. They both include exposed components which in normal usage are susceptible of damage to the components without which the structures are inoperable. The unbalanced electrode design of the Codina structures reduces their efficiency in wave detection from all directions as is necessary to ensure maximum safety. As a further drawback, the Codina structures use a transmitter and a totally separate receiver for sounding the alarm increasing the number of pieces required to make the system work, the complexity of the circuits in the system and the likelihood of malfunctions.
From an electrical standpoint the Codina structure as well as other known water detecting devices, sound an alert which can only be shut down by either turning the device off or if left unattended, by allowing the power supply to completely drain. In the case where the alarm is manually shut off, it is often easy to forget to manually turn the device on again for later use. The draining down of the batteries can on the other hand occur if the device is inadvertantly activated without anyone being present to notice the alert. Furthermore, proper testing must be done to determine whether or not the device is still operative and without such testing one might rely upon the device without realizing that it is no longer operational.